Remember non-note tree entries when reading

In order to safely edit a notes tree, NoteMap needs to retain any
non-note tree entries it read from the source tree and put them
back out into the modified tree when it commits a new version of
the note branch.

Remember any tree entries that didn't look like a note during
the parsing of the tree, so they can be put into a TreeFormatter
later when the tree writes to the repository.

Change-Id: Ia284af7e7866da35db35374c6c5869f00c857944
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>